# Fitter function for a probabilistic index model

### Description

This is the basic computing engine called by `pim`

to get the estimates for the coefficients and the variance-
covariance matrices. This function currently only spits out
these components using the sandwich estimators.

### Usage

1 2 |

### Arguments

`x` |
a model matrix with as many rows as |

`y` |
a vector with the pseudo-responses |

`link` |
a character vector with a link function |

`estim` |
a character vector or a function indicating the solver
to be used for estimating the coefficients. By default this is
the function |

`start` |
A numeric vector with the starting values for the fitting algorithm, if required. |

`vcov.estim` |
a function to determine the variance-covariance matrix.
Possibilities are |

`weights` |
currently not implemented |

`penv` |
An environment, |

`...` |
Further arguments that need to be passed to
the estimation function. The most relevant is |

### Value

A list with the following elements

- coefficients
a numeric vector with the coefficients

- vcov
a numeric matrix with the variance-covarianc matrix for the coefficients

- fitted
a numeric vector with the raw fitted values

- estim
a list with two components named

`coef`

and`vcov`

containing information on the used estimators for both.

### See Also

`model.matrix`

for how to construct a valid model matrix
for a pim, `pim`

for the general user interface